The present invention relates to a fluid for drilling, and more particularly to a drilling fluid containing an alkali metal salt of sulfoethylcellulose.
In oil well drilling. it is desired that (1) formation cuttings dug up with drilling the ground by a drill bit is removed away and the drill bit is cooled, and (2) a lubricating action is imparted to a drill stem. A drilling fluid having these functions is passed down through a drill pipe, jetted out from a nozzle of the bit and returned to the ground surface through an annular portion. Most of the drilling fluids are usually called mud, and are dispersions of clay particles swollen by hydration.
A clay-based fluid containing bentonite, attapulgite, water loss reducing agent, weighting materials, lost circulation materials and others, namely a mud used in drilling serves as a lubricant for the drill stem as a carrier for carrying formation cuttings and other solid materials such as a weighting material from the bottom hole to the ground surface and as a coolant for the drill bit, and also forms a filter cake on the well wall, whereby preventing the oil well from collapse. In order to effectively exhibit these important functions, the mud is required to always have properties such as an ability of reducing water loss an adequate viscosity and a thixotropic property. A water loss reducing agent is used for reducing the water loss of the mud. In general, it is employed in an amount of 1 to 3 pounds per barrel. It is desired that the water loss is not more than 10 ml. when the water loss reducing agent is used in such an amount. The smaller the water loss, the better. Sodium carboxymethylcellulose (hereinafter referred to as "Na-CMC") has been widely employed as a water loss reducing agent to be included in the mud. In that case, water containing mostly monovalent salts is used for forming the mud. In case of the sea water type mud, it is possible to impart the salt stability to the mud by merely raising the degree of substitution of Na-CMC. However, in recent years, the oil well is being deeper, and it is required to drill a place containing a large amount of polyvalent metal salts such as CaCl.sub.2, CaSO.sub.4 and MgCl.sub.2 in an underground layer. Also, since a mud effective for drilling through heaving shale contains Ca(OH).sub.2, CaCl.sub.2 and so on in saturation, conventionally used Na-CMC is not expected to be an effective water loss reducing agent for such a mud. This is the same even if the amount of Na-CMC is increased. Accordingly, there has been desired a water-soluble water loss reducing agent, as a mud additive, being stable and capable of exhibiting an excellent water loss reducing ability even in the presence of large amounts of polyvalent metal salts.
A clay-based drilling fluid has also a function of preventing a high pressure layer from blowing out a gas in addition to a function of cooling the drill bit and a function of carrying formation cuttings, since the various properties of the drilling fluid are largely developed by the colloidal properties such as gel strength and specific gravity of the mud. A weighting material such as barite is usually employed for raising the specific gravity of the clay-based fluid. Insoluble substances such as clay and barite included in the mud form a filter cake on the well wall during drilling down the ground. The formation of filter cake is very important from the viewpoint of well collapse prevention till reaching a producing zone such as an oil reservoir. However, after the bit reaches the producing zone, the formation of filter cake causes a serious problem, since the surface of the producing zone is covered by the filter cake, thus resulting in reduction of a permeability in the producing zone that may be said to be permanent. The clay and barite particles constituting the filter cake can be removed by conducting acid treatment at times, but often such a well damage cannot be restored even by such an expensive acid treatment, since clay and barite are low in solubility to acids. Therefore, it is necessary for preventing reduction in permeability to use a clear fluid having a specific gravity as high as sufficiently resisting the formation pressure instead of the clay-based fluid. Such a fluid is called a completion fluid. Similarly, it is desirable that a work over fluid does not cause a formation damage, since a permeable producing zone contacts the work over fluid.
An aqueous salt solution having a high specific gravity such as a saturated aqueous solution of CaCl.sub.2 has been usually employed as a completion fluid. This fluid does not cause a formation damage. The highest specific gravity of the aqueous CaCl.sub.2, solution is 1.38 g/cm.sup.3 (11.5 pounds/gallon). However, it is not a completion fluid satisfactory for all well drilling operations. In case of using a combination of CaBr.sub.2 and CaCl.sub.2, it is possible to obtain an aqueous solution having a specific gravity of 1.81 g/cm.sup.2 (15.1 pounds/gallon). Also, mixing of an aqueous solution of CaCl.sub.2 and other aqueous solutions having a high specific gravity, e.g. aqueous solutions of metal salts such as ZnBr.sub.2, PbBr.sub.2 and ZnCl.sub.2, is proposed to obtain fluids having a high specific gravity. Water-insoluble substances such as CaCO.sub.3 may be added to aqueous salt solutions for the purpose of further raising the specific gravity of the solutions and for the purpose of minimizing invasion of completion fluids into a permeable formation and fractures at the time of gun-perforation. CaCO.sub.3 is often used, since it is easily soluble in acids and also particles having various particle sizes are available.
It is necessary to add polymers to such completion fluids as a viscosifier, as a water loss reducing agent and as an agent for improving a carrying capacity. The polymers also serve as dispersants for CaCO.sub.3. They are usually employed in an amount of 0.3 to 1.5 % by weight based on the volume of a completion fluid. Polymers conventionally used for this purpose are, for instance, (1) hydroxyethylcellulose (HEC), (2) carboxymethylcellulose (CMC), (3) polyacrylonitrile, (4) xanthan gum, and (5) guar gum. In particular, HEC is widely employed. However, these high molecular additives have drawbacks. That is to say, HEC causes foaming when dissolved with agitation. CMC and polyacrylonitrile are not soluble in acids. Xanthan gum contains about 50 % of a substance insoluble in acids. Also, xanthan gum and guar gum are deteriorative to enzymes. In particular, foaming resulting from HEC is fatal for use in drilling fluids, since not only corrosion owing to oxygen occurs, but also the apparent specific gravity of fluids is lowered. Further, since foaming is of an extent such that it is not avoided by addition of antifoaming agents, the cost for the antifoaming agents cannot be disregarded. In case of certain kinds of inorganic cations included in completion fluids, polymers form insoluble materials with the cations and precipitate. Accordingly, there has also been desired a high molecular additive for completion fluids having excellent viscosity building ability, water loss reducing ability, carrying capacity and ability of dispersing water-insoluble substances such as CaCO.sub.3, and being stable in a wide pH range and scarcely toxic, and not causing formation damage.
It is an object of the present invention to provide a high molecular additive suitable for use in drilling fluid containing at least one of monovalent and polyvalent metal salts.
A further object of the invention is to provide a drilling mud having a low water loss.
A still further object of the invention is to provide a drilling mud showing a low water loss, a stable viscosity and a stable thixotropic property in a wide pH range.
Another object of the invention is to provide a completion fluid which has a sufficient viscosity, a low water loss, excellent carrying capacity and dispersing ability, and is stable in a wide pH range and does not cause formation damage.
These and other objects of the present invention will become apparent from the description hereinafter.